A quadrupole mass spectrometer using a quadrupole mass filter in a mass separator for separating ions in accordance with their mass-to-charge ratio has been widely known as a type of mass spectrometer. Typical quadrupole mass filters are composed of four cylindrical rod electrodes disposed substantially parallel to each other in such a manner as to surround the ion optical axis C. A voltage of ±(U+V·cos ωt) is applied to each of the four rod electrodes, in which a direct current voltage U and a radio-frequency voltage V·cos ωt are superimposed. This voltage forms a radio-frequency electric field and a direct current electric field in the space surrounded by the four rod electrodes. Consequently, only an ion having a specific mass-to-charge ratio is selectively allowed to pass through, and other unnecessary ions are dispersed along the way.
In one known configuration of such a quadrupole mass spectrometer, a pre-filter, which is normally shorter than a main rod electrode, is provided in the previous stage of the four main rod electrodes which compose a quadrupole mass filter (refer to Patent Documents 1 and 2 or other documents). Although a pre-filter is sometimes called a pre-rod or the like, in this specification, it is called a pre-filter as a whole, and each electrode is called a pre-rod electrode. FIG. 7 illustrates the schematic diagrams of a pre-filter 13 and a quadrupole mass filter 14: (a) is an arrangement diagram on a plane including an ion optical axis C, and (b) is an arrangement diagram on a plane orthogonal to the ion optical axis. The main objective of the pre-filter 13 is to increase the ion's passage ratio and mass resolution. Generally, the application of the voltage is controlled in the following manner: to the main rod electrodes which compose the quadrupole mass filter 14, a voltage of Vbias1±(U+V·cos ωt) is applied in which a direct current bias voltage Vbias is further added to the aforementioned voltage. On the other hand, to the pre-rod electrodes which compose the pre-filter 13, a voltage of Vbias2±V·cos ωt is applied in which a direct current bias voltage Vbias2 is added to the radio-frequency voltage component which is applied to the main rod electrodes.
As just described, the direct current bias voltage applied to the pre-filter 13 is generally and conventionally constant regardless of the mass-to-charge ratio of the target ion that should be allowed to pass through. However, this has the following problem: an ion passing through the pre-filter 13 flies, as schematically illustrated in FIG. 7(a), while periodically oscillating with a period of T=1/f [sec] for the frequency f of the radio-frequency voltage applied to the pre-rod electrodes. In the case where the frequency f is set to be constant regardless of the ion's mass-to-charge ratio, if the flight speed, i.e. the time period required to pass through the pre-filter 13, differs due to the difference of the energy that an ion has, the ion oscillation's phase at the exit of the pre-filter 13 becomes different. Generally, at the entrance of the quadrupole mass filter 14, an ion efficiently enters the quadrupole mass filter 14 in the case where the ion oscillation's phase satisfies a predetermined condition. Depending on the mass-to-charge ratio of an ion, the phase of the oscillation at the entrance of the quadrupole mass filter 14 may not satisfy the aforementioned entry condition, which causes a relatively large loss. As a result, in performing a mass scan across a predetermined mass range for example, the detection sensitivity may differ depending on the mass.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. H07-240171    [Patent Document 2] Japanese Unexamined Patent Application Publication No. H11-25904